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WASHINGTON (CNN) -- Defending the U.S. government's response to Hurricane Katrina, Homeland Security Secretary Michael Chertoff argued Saturday that government planners did not predict such a disaster ever could occur.

But in fact, government officials, scientists and journalists have warned of such a scenario for years.

"That 'perfect storm' of a combination of catastrophes exceeded the foresight of the planners, and maybe anybody's foresight," Chertoff said.

He called the disaster "breathtaking in its surprise."

But engineers say the levees preventing this below-sea-level city from being turned into a swamp were built to withstand only Category 3 hurricanes. And officials have warned for years that a Category 4 could cause the levees to fail. (See video of why the levee's breech was devastating -- 1:53)

Katrina was a Category 4 hurricane when it struck the Gulf Coast on August 29.

Last week, Michael Brown, head of the Federal Emergency Management Agency, told CNN his agency had recently planned for a Category 5 hurricane hitting New Orleans.

Speaking to "Larry King Live" on August 31, in the wake of Katrina, Brown said, "That Category 4 hurricane caused the same kind of damage that we anticipated. So we planned for it two years ago. Last year, we exercised it. And unfortunately this year, we're implementing it."

Brown suggested FEMA -- part of the Department of Homeland Security -- was carrying out a prepared plan, rather than having to suddenly create a new one.

Chertoff argued that authorities actually had assumed that "there would be overflow from the levee, maybe a small break in the levee. The collapse of a significant portion of the levee leading to the very fast flooding of the city was not envisioned."

He added: "There will be plenty of time to go back and say we should hypothesize evermore apocalyptic combinations of catastrophes. Be that as it may, I'm telling you this is what the planners had in front of them. They were confronted with a second wave that they did not have built into the plan, but using the tools they had, we have to move forward and adapt."

But New Orleans, state and federal officials have long painted a very different picture.

"We certainly understood the potential impact of a Category 4 or 5 hurricane" on New Orleans, Lt. General Carl Strock, chief of engineers for the U.S. Army Corps of Engineers, said Thursday, Cox News Service reported.

Reuters reported that in 2004, more than 40 state, local and volunteer organizations practiced a scenario in which a massive hurricane struck and levees were breached, allowing water to flood New Orleans. Under the simulation, called "Hurricane Pam," the officials "had to deal with an imaginary storm that destroyed more than half a million buildings in New Orleans and forced the evacuation of a million residents," the Reuters report said.

In 2002 the New Orleans Times-Picayune ran a five-part series exploring the vulnerability of the city. The newspaper, and other news media as well, specifically addressed the possibility of massive floods drowning residents, destroying homes and releasing toxic chemicals throughout the city. (Read: "Times-Picayune" Special Report: Washing away)

Scientists long have discussed this possibility as a sort of doomsday scenario.

On Sunday, a day before Hurricane Katrina made landfall, Ivor van Heerden, director of the Louisiana State University Public Health Research Center in Baton Rouge, said, "This is what we've been saying has been going to happen for years."

Also on Sunday, Placquemines Parish Sheriff Jeff Hingle referred back to Hurricane Betsy -- a Category 2 hurricane that struck in 1965 -- and said, "After Betsy these levees were designed for a Category 3."

He added, "These levees will not hold the water back."

But Chertoff seemed unaware of all the warnings.

"This is really one which I think was breathtaking in its surprise," Chertoff said. "There has been, over the last few years, some specific planning for the possibility of a significant hurricane in New Orleans with a lot of rainfall, with water rising in the levees and water overflowing the levees," he told reporters Saturday.

That alone would be "a very catastrophic scenario," Chertoff said. "And although the planning was not complete, a lot of work had been done. But there were two problems here. First of all, it's as if someone took that plan and dropped an atomic bomb simply to make it more difficult. We didn't merely have the overflow, we actually had the break in the wall. And I will tell you that, really, that perfect storm of combination of catastrophes exceeded the foresight of the planners, and maybe anybody's foresight."

Chertoff also argued that authorities did not have much notice that the storm would be so powerful and could make a direct hit on New Orleans.

"It wasn't until comparatively late, shortly before -- a day, maybe a day and a half, before landfall -- that it became clear that this was going to be a Category 4 or 5 hurricane headed for the New Orleans area."

As far back as Friday, August 26, the National Hurricane Center was predicting the storm could be a Category 4 hurricane at landfall, with New Orleans directly in its path. Still, storms do change paths, so the possibility existed that it might not hit the city.

But the National Weather Service prediction proved almost perfect.

Katrina made landfall on Monday, August 29.

Tens of thousands of people in New Orleans who did not or could not heed the mandatory evacuation orders issued the day before the storm made landfall were left in dire straits.

"I think we have discovered over the last few days that with all the tremendous effort using the existing resources and the traditional frameworks of the National Guard, the unusual set of challenges of conducting a massive evacuation in the context of a still dangerous flood requires us to basically break the traditional model and create a new model -- one for what you might call kind of an ultracatastrophe," Chertoff said.

He vowed that the United States "is going to move heaven and earth" to rescue those in need.